Hot subluminous stars 500pc volume-limited sample

The overall mass distribution peaks at 0.48_-0.10_^0.14^M_{sun}_, while hot subdwarfs below the EHB peak at 0.43_-0.09_^0.11^M_{sun}_, supporting a scenario of non- or semi-degenerate helium ignition for these objects characteristic of intermediate-mass stars. Interpolation of EHB and post-EHB tracks yields theoretical mass distributions consistent with our estimates based on SED and parallax. By assuming a mass range between 0.40 and 0.50M_{sun}_ during the interpolation, we further find that the post-EHB birthrate in our sample is 2-3 times higher than the EHB birthrate, which may suggest overestimated EHB lifetimes in theoretical tracks or contamination from other formation and evolutionary channels.

Our kinematic analysis shows that 86+/-2% of the stars belong to the Galactic thin disk, with 13+/-1% and 1+/-1% associated with the thick disk and halo, respectively. The below-EHB population is exclusively found in the thin disk, the only Galactic population young enough to harbour intermediate-mass stars. The below-EHB population seems to be absent in other large samples, which generally include more thick disk and halo members.

These findings suggest that non-degenerate formation channels may play a more prominent role in the Galactic disk than previously thought.